The bonus game is something that you really want to play in the slots games because it can be very profitable.
This feature is activated by a special combination of symbols on the game area. The symbol could be a specific bonus symbol or another type of combination. The bonus game can have a different setup where you spin on the wheel of Fortune as in the slot Mega Fortune från NetEnt or other variants where you shall click on coffins that contains winning amounts and the bonus game goes on until you find a bomb symbol.
In the slot Book of Dead from PlayN Go the bonus game is activated of a combination of three or more books on the game area. After that a specific bonussymbol is rolled on the screen and decided to be the bonussymbol in the bonusgame and then the freespin/bonusgame starts and then that active symbol is generating the wins with expanding symbols over the reels. During these bonus games it is also possible in some slots to retrigger the bonusgame and the vendors have constructed the gameplay a little bit different from each slot so make sure to check the information tab in the slot to know how the bonus game works.
We can say that it is definitely very interesting to get the bonus game and depending what kind of game we are talking about and what kind of volaitility it can take a lot of spins until the bonus game is activated, but when it does it could be very profitable. Remember that to activate the bonus game has nothing to do with knowledge and you can not affect when it will happen so this is just a random feature in the game.